1. Field of the Invention
The present invention is generally related to a Sn—Sb—Ni ternary compound, and more particularly to a Ni(Sn, Sb)3 skutterudite compound and preparation method thereof.
2. Description of the Prior Art
Soldering is an important bonding method in electronic products and is also an important technique in electronic packaging. Low-temperature solder is used to bond different substrates and elements. When the soldering temperature is higher than the melting point of the solder, the melted solder wets the substrate and reacts with the substrate. Then, the temperature of the solder drops and the solder solidifies to form a soldering joint. The process of the electronic packaging starting from assembling one or more than one integrated chips with a package structure till completing packaging an electronic product includes various soldering processes. In order to prevent the soldering joint in the previous process from being damaged or re-melted, the solder having a higher melting point should be used in the earlier stage of the packaging process and the solder having a lower melting point should be used in the higher level or the later stage of the packaging process. Besides, in order to prevent the solder from directly contacting with the electronic element to damage the electronic element, before the soldering process, under bump metallurgy (UBM) should be deposited on the electronic element.
Since Sn—Pb solder has good wetting and mechanical properties and also does not react with the substrate easily, the Sn—Pb solder has been used in electronic industry for a long time and its processing methods have been well developed. However, since lead (Pb) is hazard to human health and environment, most of electronic products nowadays are lead-free. Recently, development and research on the lead-free solder has been drawn great attention throughout the world and the factors including a proper melting point, high heat dissipation coefficient (<20˜30 W/mK), low resistance (<0.0001 Ω·cm), good wetting property with various substrates, being unable to react with substrates vigorously, and low production cost should be considered. The potential and commonly seen solder includes, for example, high temperature solders Au-20 wt % Sn and Sn-5 wt % Sb. Au-20 wt % Sn-5 wt % Sb has been considered having the most potential in the future. The Sn—Sb solder has a disadvantage of having a relatively low melting point but Au—Sn has a high cost problem. Besides, nickel (Ni) has terrible reactivity with solders among known materials but Ni is a commonly used material for a diffusion barrier layer. When a solder is reflowed on UBM, its wetting layer dissolves quickly into the solder to form a soldering joint of Sn—Sb/Ni.
When the solder and the substrate are in contact with each other, elements diffuse and react to form an intermetallic phase because the chemical potential of each element is different from each other. The intermetallic phase having an appropriate thickness can increase the adhesiveness between the solder and the substrate but a thick interface may cause weakness of the mechanical property of the soldering joint to lower the reliability of a product because the intermetallic phase is brittle.
On the other hand, since energy resources of the earth are depleting, renewable green energy sources, such as solar energy, wind power, biomass, etc., become important. Although waste heat recovery is not considered important yet, according to reports from department of energy of the United States, at least 5% of energy will be obtained from waste heat recovery in the United States till 2020. Waste heat recovery is worth further research. Thermoelectric materials can convert thermal energy into electric energy or can be used as a cooler if supplied with electric energy. Compared to a refrigerator, since thermoelectric materials do not need a coolant and can convert waste heat into electric energy, thermoelectric materials are very environmental-friendly. The property of a thermoelectric material is often represented by a dimensionless equation:ZT=(S2Tσ)/κ
where ZT: thermoelectric figure of merit; S: Seebeck coefficient; σ: electrical conductivity; κ: thermal conductivity.
In 1992, Jet Propulsion Laboratory of US proposed the concept of using a skutterudite compound as a thermoelectric material having a basic chemical formula of MX3 where M=Ni, Co, Fe, Rh, Ir and X═P, As, Sb. Its structure contains vacancies and the thermoelectric figure of merit is significantly improved after the vacancies are filled with metal atoms. The doping or substitution method is often used to change the composition of the skutterudite compound in order to optimize the thermoelectric figure of merit.
Ni(Sn1-x, Sbx)3 is a thermodynamically stable ternary phase. According to previous research, Ni(Sn1-x, Sbx)3 is a stable ternary phase formed from NiSb3 by doping or substituting with Sn and Ni(Sn1-x, Sbx)3 is a skutterudite compound and a good thermoelectric material. Therefore, a Sn—Sb—Ni alloy is also a potential material as a negative electrode of a lithium ion battery. Currently, the material used as the negative electrode of a lithium ion battery is carbon but the theoretical charge capacity of carbon is only about 372 mAh/g. Due to such a limit, a new material should be used to replace carbon in order to increase the charge capacity of the negative electrode. Theoretically, any metal capable of forming an alloy with lithium can be used as the material of a negative electrode. However, the volume variation of pure metal is relatively large during the process of forming an alloy with lithium and thus the mechanical stability of the material decreases after repeatedly charging and discharging so that the material gradually becomes powders and ineffective. If an intermetallic compound is used instead of pure metal, the volume variation can be effectively suppressed and the cycle property of the material can be improved.
Currently, the research on the negative electrode alloy material comprises two types of intermetallic compounds, one of which is active/active type and the other of which is active/inactive type. The active/active binary intermetallic compound comprises two elements that can react with lithium to form an alloy. Since the electric potentials of the two elements, such as Sn—Sb, Sn—Ag, etc., reacting with lithium are different, the unreacted element can be used as a buffer during volume variation. As for the active/inactive type, an “inactive” element (or metal) means a metal that does not react with lithium. Compared to the active/active type, the active/inactive type, such as Sn—Ni, Sn—Cu, etc., can suppress the volume variation more effectively but has a lower theoretical charge capacity. In the research of using a metal alloy as the material for a negative electrode, Sn and Sb are important because they have high theoretical charge capacity (994 and 660 mAh/g). Ni is an inactive metal to lithium ions but has ductility to buffer the volume inflation of the electrode. Therefore, the active/active alloy can make the electrode have high charge capacity and the characteristic of reacting with lithium ion at different electric potentials can slightly suppress the volume variation. On the contrary, the inactive metal almost does not react with lithium ion and has good ductility to significantly suppress the volume variation to solve the problem of low cycle stability and also to significantly increase the cycle lifetime of a metal electrode.
Fabrication of electrode materials can be divided into two types: powder and film types. The following equation shows using the carbon hot-reduction method to fabricate Sn—Sb—Ni ternary alloy powders as the material for a negative electrode of a lithium ion battery:2xSnO2+Sb2O3+2NiO+(4x+5)C→2SnxSbNi+(4x+5)CO↑,where x=1, 2 and 3.
The above method uses 800° C. calcination to form the Sn—Sb—Ni ternary alloy and lets the alloy cool to room temperature. The Sn—Sb—Ni ternary alloy synthesized by the above method has various phases and the crystal structure of each phase is different from each other so that the arrangement of each phase in the alloy is irregular and the alloy can easily become powders during charging/discharging processes to shorten the lifetime of the battery. The conventional fabrication method is carried out based on thermodynamic theorem but the resulting alloy usually contains thermodynamic multiple phases, the processing time is long, and the production cost is high.
In conclusion, from the reports of research on the material for a negative electrode of a lithium ion battery, using Sn—Sb—Ni is a new technology. Therefore, how to fabricate a Sn—Sb—Ni ternary compound at low cost is important for the industry.